Mitochondrial RNA Splicing in Neurospora crassa

Abstract

The mtDNAs of lower and higher eukaryotes contain essentially the same complement of genes coding rRNA and tRNA species and a limited number of inner membrane polypeptides, but they show considerable divergence in size, structure, and genetic organization (Gillham 1978). In fungi the genes coding the small and large mitochondrial rRNAs are separated by stretches of DNA ranging from 1.3 kbp to 30 kbp, whereas in animal mtDNAs the corresponding genes are separated by less than 200 bp (Boynton et al. 1980; Grant and Lambowitz 1982). In addition, some yeast and fungal mtDNAs, but not animal mtDNAs, contain intervening sequences in the genes that code certain polypeptides (Borst and Grivell 1978) and the large rRNA (Boynton et al. 1980; Grant and Lambowitz 1982).

Although the basic features of mtDNA gene organization have now been elucidated for some animals and lower eukaryotes, there is still relatively little information about the regulation of mitochondrial gene expression in any experimental system. Such questions are best addressed by biochemical-genetic approaches in lower eukaryotes. We have been developing an experimental system focusing on nuclear mutants of Neurospora crassa that are defective in splicing the mitochondrial large rRNA. The mutants open unique experimental approaches for studying RNA splicing and its role in gene expression. In this paper we review the salient features of the experimental system and the mutants. In addition, we present evidence, from the characterization of one mutant, that RNA splicing in Neurospora mitochondria may be subject to regulation by the rate of electron...